In impact printing, the repetition rate of the print hammer mechanism is limited by the time it takes the hammer mechanism to come to rest, i.e. to settle at its initial or rest position, after printing impact occurs. This has been a long standing problem particularly with inertial type print hammers. In such mechanisms, a print hammer or other impact element is propelled in free flight at high velocity to effect printing on a print medium which may consist of ink ribbon and paper. The resulting impact causes the hammer to rebound with a substantial portion of the original kinetic energy which must be dissipated to bring the hammer to rest. Both registration and print density are adversely affected if the actuated elements of the hammer mechanism are not settled and restored to their rest position before the next printing operation occurs. A subsequent hammer mechanism operation prior to settling varies both the magnitude and timing of the transmitted impact force so that variable density and misregistration result in the print.
Shorter settling times require rapid removal of rebound kinetic energy from the print mechanism. Various solutions have been proposed in the past. They are generally complex, expensive, ineffective or too slow.
One technique has been to have the rebounding hammer mechanism elements strike against a backstop comprised of energy absorbing materials, such as elastomers or butyl rubber, and against which the rebounding elements can come to rest. Prominent disadvantages are that the settling time is not radically shortened and, after the energy absorbing material receives repeated beating, it changes its energy absorption characteristics and dimensions. Eventually the original rest or home position of the print mechanism is changed and adjustment or replacement of all or part of the print mechanism is required to maintain good print quality. Examples of such mechanisms are shown in U.S. Pat. Nos. 3,241,480; 3,675,172 and 4,496,253.
Another technique has been to apply a frictional drag to the hammer element as in U.S. Pat. Nos. 4,329,921 and 2,696,782 but this causes undesired rapid wear of the components.
A different method has been to trap or block the rebounding hammer as it returns from impact with the type, such as shown in U.S. Pat. Nos. 3,143,064, 2,696,782 and 2,353,057. In a further arrangement, a print hammer carries a freely movable weight which is impelled against the hammer as a result of the hammer impacting the type and again when the hammer is arrested at its home position. While this arrangement prevents the hammer from making a second impression, it does not prevent the hammer from rebounding off the backstop and does not appreciably reduce settle time. Furthermore, greater energy must be expended to operate the hammer as a consequence of the added weight carried by the hammer. Examples of this arrangement are shown in U.S. Pat. Nos. 2,616,366 and 2,625,100.